NEC Ungrounded Systems

[JensenDrive]

NEC 250.36 covers high impedance grounding, and 250.36.a states that the ground reference has to be to a neutral. It says:
"The grounding impedance shall be installed between the grounding electrode conductor and the system neutral. When a neutral is not available, the grounding impedance shall be installed between the grounding eledctrode conductor and the neutral derived from a grounding transformer."

This wording broaches on disallowing completely ungrounded delta or wye secondaries, and further disallows high impedance corner grounding of deltas. I know most people do not like high impedance grounded systems, but if you can ignore that issue, does anyone want to comment if the NEC has a valid reason for the above paragraph?

 

[davidbeach]

I don't know where your "most people do not like high impedance grounded systems" comes from, but I doubt it is true if "most people" is limited to those who understand the systems well enough to comment on them.

But anyway...

The code lists systems not required to be grounded, these can continue to be ungrounded, though there is not really any such beast. Nominally ungrounded systems are actually capacitively grounded and that is in many cases the worst possible grounding method. High impedance grounding is far superior in that it has all of the advantages usually assigned to "ungrounded" systems without the transient overvoltage problems brought on by arcing faults.

The prohibition on high impedance (almost always high resistance) grounding of delta systems by grounding a corner is quite logical. The rules for corner grounded systems are all based on the grounded corner being at ground; two fuses instead of three and so forth. If that phase were high resistance grounded rather than solidly grounded it would raise to phase-to-phase voltage above ground rather than being very close to ground; a definite safety problem. A high resistance grounded system with the neutral grounded through the resistor will operate with all phase-to-ground voltages balanced during normal operations and the two unfaulted phases will raise to phase-to-phase voltage to ground when the third phase has a ground fault.

 

[JensenDrive]

"The rules on corner grounded systems..." Are you reference a code section that you can point me to?

I did not follow your comment "If that phase were high resistance grounded rather than solidy grounded it would raise the phase-to-phase voltage above ground..." I am having a hard time envisioning the circuit you are describing. Can you elaborate a little?